Until recently it has not been possible to develop criteria to distinguish between the normal loss of skeletal mass with age and that occurring in osteoporosis. With the use of total body neutron activation and analysis, it has become possible, for the first time, to measure total body calcium (TBCa) directly and hence to measure skeletal mass. This technique along with photon absorptiometry will be used to measure TBCa in cross-sectional study of normal white and black indivduals. The normal skeletal mass can be predicted for an individual on the basis of sex, age, height and lean body mass. Thus, the difference between the measure and the predictive value can be determined for any individual. The basic assumption underlying the measurement of bone mass for the evaluation of osteoporosis is that there exists a relationship between the level of bone mineral and osteoporosis. If the hypothesis that the structural integrity of the skeleton is associated with a critical level of mineral is accepted, it is essential to determine this threshold value and to establish the degree of osteoporosis in terms of deficiency of calcium from a normal value. Evaluation will also be made of the effect of physical activity on the rate of skeletal loss in a geriatric population.